The present invention concerns a mainspring for a timepiece coiled inside a barrel at an angle .alpha. of n2.pi. radians, said mainspring having spiral form and a rectangular section of consistant width, the first end of the spring being hooked to a core integral with the barrel-arbor, at least a first portion of the spring extending from the wall of the barrel towards the core, exhibiting an increasing thickness e.
Swiss patent CH-A-375275 discloses a timepiece mainspring whose force gradually reduces going from the internal end to the external end for the purpose of obtaining a more regular barrel motor force and, consequently, a quasi constant amplitude of the balance.
The document cited above states that springs whose section gradually diminishes from the internal end to the external end are known. This section variation may be obtained by gradually changing, from one end to the other, notably the thickness of the spring.
Patent FR-A-1583064 discloses a rolling-mill enabling such a spring to be fabricated. It comprises two rolling cylinders able to move apart from each other under the action of an eccentric.
The first document cited above discloses a gradual reduction in thickness as a function of the coiling angle of the spring, but does not state the law according to which this reduction takes place. If it concerns a linear variation, it will be seen later that this law is not capable of providing a constant torque.